Continue to assume None for un-accessed attribute on persistent

Continue to assume None for un-accessed attribute on persistent
object during m2o fetch

Fixed regression in new relationship m2o comparison logic first introduced
at :ref:`change_4359` when comparing to an attribute that is persisted as
NULL and is in an un-fetched state in the mapped instance.  Since the
attribute has no explicit default, it needs to default to NULL when
accessed in a persistent setting.

Fixes: #4676
Change-Id: I17160c30131187c735f025a785ff0276a246f6bb